The present invention relates to spark gap devices and particularly to a triggered spark gap discharger of improved design and operation.
A need exists for devices capable of rapidly switching or discharging high currents with a relative low voltage drop thereacross. Transistors and vacuum tubes are capable of handling currents of a few amperes. Cold cathode trigger tubes and thyratrons are useful in switching several hundred amperes. At higher voltages and currents of a few thousand amperes, ignitrons are required. For very high operating voltages and currents, triggered spark gaps come into use.
A conventional triggered spark gap discharger consists essentially of a pair of electrodes spaced far enough apart such that the voltage applied across the electrodes is insufficient to electrically break down the gap therebetween. The gap remains a very good insulator at voltages below its hold-off value. When it is desired to initiate the flow of current, a method must be provided to cause sufficient ionization of the gas between the electrodes to allow the gap to breakdown. This may be accomplished by a sudden increase of the voltage across the gap, a sudden reduction in the gap spacing, a sudden reduction in gas density, natural radioactive irradiation of the gap, ultraviolet irradiation of the gap, a heated filament in the gas dielectric, distortion of the electric field of the gap, or injection of ions and/or electrons into the gap.
Commercially available triggered spark gap dischargers, commonly known as triggertrons, consist of a pair of hemispherical primary electrodes with an axial trigger probe in one electrode. Upon application of a trigger pulse, an auxiliary spark is generated inside the gap between the trigger probe and its associated primary hemispherical electrcde, or the other primary electrode, depending upon the gap design and the polarity of the electrodes. The auxiliary spark provides a source of electrons and ions and forms a low-density region due to the energy dissipated by the trigger spark. The combination is mounted in a sealed chamber filled with an ionizable gaseous medium.
One application for such a triggered spark gap discharger is in the protection of klystrons from damage due to internal short circuits by preventing occasional breakdown currents therein from reaching values of more than a few hundred amperes. Since this current may reach a few hundred amperes in about 20 nanoseconds, the triggered spark gap must fire reliably and very promptly if it is to protect the klystron adequately. Failure of the trigger spark gap to fire can lead to damaging currents of several thousand amperes through the klystron.
The triggering circuit may be such that a large trigger signal, in the order of sixty percent or more of the main gap voltage, is available on the triggering probe before the flashover of the klystron lowers the main gap voltage appreciably. Thus, the triggering circuit provides a large and fast trigger signal while the voltage on the main electrodes is still high. Very fast and reliable triggering of the spark gap is required in response.
Since flashover in the klystron may occur while the supply voltage is being increased from zero, it is also necessary that the triggered spark gap discharger work at reduced main gap and trigger voltages. The triggered spark gap discharger must, in a particular application, operate within about 120 nanoseconds at 60 to 100 kilovolts across the main gap electrodes and within 300 nanoseconds at 55-59 kilovolts thereacross. Since the current increases rapidly with time, if the gap fails to fire, the current in the klystron reaches nearly four times the permitted level within one microsecond. Thus, it is important that the triggered gap operate reliably and fast to as low a voltage as possible.
Newly manufactured triggered spark gap dischargers have sharp-edged tips to the trigger probes, and these edges easily emit electrons by field emission. As the devices age, however, the tips of the trigger probes or pins become rounded by erosion by the sparks, and there is then typically a delay before an electron is available to initiate a trigger spark. This delay causes performance of the dischargers to deteriorate, with consequent deleterious effects upon associated devices and/or circuitry.